1. Field of the Invention
The present invention relates to a radiographic image detector that detects an image of a subject from radioactive rays penetrating through the subject.
2. Description of the Related Art
In the medical field, radiography using radioactive rays like x-rays has been well known as a diagnostic method. An x-ray radiography system generally consists of an x-ray source for radiating x-rays, and an x-ray image detector for detecting an x-ray image of a subject from x-rays penetrating through the subject. Among those used practice, there is an x-ray image detector that adopts a flat panel detector (FPD) having a TFT (thin film transistor) active matrix substrate, in which pixels having a thin film transistor (TFT) each are arranged in an array for accumulating signal charges in the respective pixels corresponding to the incident amounts of the x-rays. The FPD outputs the accumulated signal charges in the form of digital image data representative of the x-ray image of the subject. There are also x-ray image detectors that use a radiographic film or an imaging plate.
Unlike these image detectors, the FPD type x-ray image detector should be controlled to synchronize its charge accumulating operation with the time of x-ray radiation from the x-ray source. For this purpose, a control device for the x-ray image detector, like a console, is configured to receive a start-of-radiation signal from a radiation starting switch when the radiation starting switch is operated to actuate the x-ray source to start x-ray radiation. The control device transfers the start-of-radiation signal as a synchronizing signal to the x-ray image detector, upon which the x-ray image detector starts accumulating signal charges for imaging.
However, the x-ray image detector may be equipped with such interfaces that are incompatible to interface standards (standards of cables and connectors, format of the synchronizing signal, etc.) of the control device for the x-ray source of the radiography system, for example, because of different manufacturers. To solve this problem, various methods of detecting the start of x-ray radiation from the x-ray source by the x-ray image detector itself in order to synchronize it with the x-ray source have been suggested.
In an x-ray image detector disclosed in United States Patent Application Publication No. 2002/101527 (corresponding to Japanese Patent Laid-Open Publication No. 2002-181942), a detection element for detecting the start of x-ray radiation is disposed outside an imaging area in which pixels are arranged in a matrix. The output of the detection element is monitored to detect the start of x-ray radiation.
United States Patent Application Publication No. 2003/086523 (corresponding to Japanese Patent Laid-Open Publication No. 2003-126072) discloses an x-ray image detector that has a radiation detecting element for detecting the start of x-ray radiation is disposed in an area of the FPD where x-rays from an x-ray source enter without penetrating through the test subject. The start of x-ray radiation is determined based on a surge in output of the radiation detecting element.
An x-ray image detector disclosed in U.S. Pat. No. 6,797,960 (corresponding to Japanese Patent Translation Publication No. 2002-543684) predetermines a threshold value with regard to dark charges which will be accumulated in pixels due to dark current generated in the FPD even without incidence of x-rays. This x-ray image detector compares the accumulated charges with the predetermined threshold value to judge that the x-ray radiation is started when the accumulated charges get over the threshold value.
United States Patent Application Publication No. 2008/288740 (corresponding to Japanese Patent Laid-Open Publication No. 2008-507796) discloses an x-ray image detector that reads out image data periodically at a predetermined frame rate in advance to the start of x-ray radiation. Depending on whether differences in pixel level between two successive image frames are above a threshold level or not, the start of x-ray radiation is determined in this prior art.
The detection method suggested in United States Patent Application Publication No. 2002/101527 (corresponding to Japanese Patent Laid-Open Publication No. 2002-181942), however, has a problem that the detection element disposed outside the imaging area may be out of an irradiation range by the x-rays when it is confined within the imaging area. In that case, the detection element cannot detect the x-rays from the x-ray source so that the FPD will not start the charge accumulating operation even after the start of x-ray radiation. As a result, the image detector cannot detect any x-ray image while the test subject is exposed to the x-rays wastefully.
According to other detection methods of the above-mentioned prior arts, at least a pixel inside the imaging area is utilized for detecting the start of x-ray radiation. Therefore, these prior arts do not have the same problem as the first mentioned prior art. However, because the signal charges accumulated upon the x-ray radiation must be read out from the pixels in order to detect the start of x-ray radiation, the x-rays radiated in a first stage of a radiation time will be used merely for detecting the start of radiation and will not be served for imaging. This is a waste of x-rays.